Porous inorganic compounds having uniform mesopores have larger pores than conventional oxides such as zeolite, and application of those compounds to catalyst carrier, separation adsorbent, fuel battery and sensor has been studied.
As for a process for preparing such an oxide having uniform mesopores, a process utilizing control of the structure of an inorganic material by the use of an organic compound has been paid attention because an oxide of novel shape and structure can be obtained. In particular, an oxide having uniform mesopores, that is synthesized by utilizing self organization of an organic compound and an inorganic compound, is known to have a larger pore volume and a larger surface area than conventional oxides such as zeolite.
As a process for preparing an oxide having uniform pores utilizing self organization of an organic compound and an inorganic compound, a process comprising subjecting a silica gel and a surface active agent to hydrothermal synthesis reaction in a heat-resistant closed vessel to prepare such an oxide is described in, for example, WO91/11390. In Bull. Chem. Soc. Jp., Vol. 63, p. 988 (1990), a process comprising subjecting kanemite that is a kind of a layered silicate and a surface active agent to ion exchange to prepare such an oxide is described.
On the other hand, the oxide having uniform mesopores has a defect that the oxide is liable to adsorb moisture because of its large pore volume and surface area. That is to say, the oxide having uniform mesopores prepared as described above has a large pore volume and contains a great number of hydroxyl groups present on the pore surfaces. Therefore, the oxide has high moisture adsorption properties, and the structure of the oxide is changed by the adsorbed water or the periodic structure of the pores is disintegrated.
Many patents to improve the moisture adsorption properties have been applied so far. For example, it is described in Japanese Patent Laid-Open Publication No. 14413/1981 that an organosilicon halide compound as a starting material is allowed to react with SiO2 in an organic solvent to develop water repellency. In this case, the organic group imparts water repellency to SiO2.
Surface treatments of silica and a silica gel are described in various publications. In Japanese Patent Laid-Open Publication No. 181715/1983, treatment with an organosilane halide and water vapor is described; in Japanese Patent Laid-Open Publication No. 295226/1986, treatment with silicone or the like is described; in Japanese Patent Laid-Open Publication No. 59415/1990, bonding to a hydrophobic organic group is described; in Japanese Patent Laid-Open Publication No. 107502/1990, treatment with a fluorinating agent in the presence of water is described; in Japanese Patent Laid-Open Publication No. 196342/1995, treatment by immersion of a substrate in a solution obtained by adding NH4F to a water-based solution of alkoxysilane is described; and in Japanese Patent Laid-Open Publications No. 157643/1996, No. 242717/1997, No. 25427/1998 and No. 140047/1998, treatment of an inorganic oxide with a fluorine-containing organosilicon compound as a surface treating agent is described. All of these methods have improved moisture adsorption properties of silica.
Further, in EP0799791, treatment with a silicone oil having an epoxy group or with an amine compound having an amino group is described; in Chinese Patent No. 1,072,654, treatment using amine or pyrrolidone is described; and in U.S. Pat. No. 4,164,509, sulfonic acid treatment is described. Moreover, in Japanese Patent Laid-Open Publication No. 92621/1994, treatment comprising hydrolyzing tetraethoxysilane and coating a substrate with the hydrolysis solution is described; in U.S. Pat. No. 4,569,833, treatment by contact with SiF4 to improve water repellency is described; and in U.S. Pat. No. 4,054,689, treatment by contact with a HF gas to improve water repellency is described.
The above methods, however, are all surface treatments of silica, and it is difficult to homogeneously treating inside surfaces of pores of the porous silica. In addition, they are not satisfactory as methods to improve moisture adsorption properties of the porous materials from the viewpoint of application of the resulting silica to light functional materials or electron functional materials, because there are disadvantages such that the treatment in water disintegrates the pore structure, the treatment with an organic material results in low heat resistance, and the F treatment by the contact with a gas results in only a temporary effect.
In Materials Letters 42 (2000), pp. 102-107, a process for preparing water-repellent porous silica having uniform pores, comprising dropwise adding a HF solution to a caustic soda solution of silica and performing hydrothermal synthesis is described. From the porous silica prepared by this process, however, any film cannot be formed. In addition, Na remaining in silica hinders application of the silica to a light functional material or an electron functional material.
On the other hand, films comprising oxides having uniform mesopores have been proposed recently, and application of those films to light functional materials or electron functional materials has been highly expected. For example, in Nature, Vol. 379, p. 703 (1996), a process comprising placing a mica board in a solution essentially consisting of tetraalkoxysilane and a surface active agent to form a film on a surface of the mica is described; in Nature, Vol. 381, p. 589 (1996), a process for forming a film on a liquid level of a solution essentially consisting of tetraalkoxysilane and a surface active agent is described; and in Science, Vol. 273, p. 768 (1996), a process for forming a film on an interface between an oil layer containing tetraalkoxysilane and a water layer containing a surface active agent is described. These processes, however, industrially have a problem that a long period of time is necessary for the formation of a film and a large amount of a powder is produced as a by-product together with the film.
In Japanese Patent Laid-Open Publication No. 194298/1997, a process comprising coating a substrate with a solution essentially consisting of tetraalkoxysilane and a surface active agent to form a film having pores regularly disposed is disclosed. In WO99/37705, a process for forming a film, which comprises converting a surface active agent into an amphiphatic block copolymer to make pores large, is disclosed. These processes are industrially useful because films are produced for a short period of time. In the resulting porous films having uniform pores, however, gradual variation of the structure or disintegration of the periodic structure of pores takes place because of the aforesaid moisture adsorption properties, and thereby the conductivity becomes high. Thus, these porous films have a problem when they are applied to light functional materials or electron functional materials.
Accordingly, development of a highly water-repellent film having uniform pores has been eagerly desired.